The surname Kavanagh or Cavanaugh and the other variants of the name are derived
from the adjectival Irish Gaelic name Caomhánach. This was the name applied to Domhnall, eldest son of the 12th century King of Leinster
Diarmait mac Murchada (Dermot MacMurrough).

Domhnall was fostered, according to Irish custom, by the family attached to the monastery of St. Caomhan at Kilcavan in the Barony of
Gorey, County Wexford. He thus became known as Caomhánach.
His brother Eanna became known as Eanna
Ceinnsealach, also an adjectival name derived from the Clan and its territory. Eanna became the progenitor of
the Kinsella Clan.

The word Caomh signifies "gentle" or "comely" in Gaelic.

As an incentive to the Norman commander Richard de Clere, Earl of Strigoil (and previously Earl of Pembroke, a title which Henry II had deprived him of) Diarmaid McMurrough had given him his daughter Aoife in marriage. De Clere was commonly known as "Strongbow"

Despite the fact that under Irish (Brehon) law, the Kingship could not be passed on this manner via marriage, Strongbow subsequently
used this marriage to attempt to establish a claim on the kingship of Leinster, following the death of King
Dermot MacMurrough in 1171AD.

However the Irish chiefs adhered to the traditional Gaelic legal system and
Domhnall Caomhanach was subsequently elected as King.

While the exact place and manner of Domhnall's death is unclear, the general opinion of historians is that he was assassinated at the behest
of the Normans as he was drumming up support for war against them.